As the demand for higher performance electronics continues to push data rates beyond 1 GHz, there is a growing trend to use differential signal protocols in electronics. Yet, there is a lack of appropriate test equipment being developed to properly characterize such signals.
Specifically, the majority of developing differential signal protocols are developed around the concept of considering only the differential portion of the signal and suppressing and ignoring the common-mode portion of the signal. By doing so, shifts in common-mode voltage, balanced impedance discontinuities in interconnect, and noise from outside sources can often be of little consequence to the serial link performance. Accordingly, many transmission line drivers and receivers provide good differential impedance match between the true and complement signal while the common-mode impedance match is often quite poor.
In contrast, the vast majority of available test equipment is designed with coaxial connections that are inherently best suited for single-ended signal protocols. Through the use of two such coaxial connectors, the equipment can provide or capture differential signals by considering only the difference between these two connection points through internal circuitry. With the use of single-ended connectors, the test equipment typically terminates the connections with 50 Ohm resistors to ground. Looking at the difference in potential between the true and complement ports, a 100 Ohm series resistance can be measured and is sometimes adequate termination for the incoming differential signal.
The disparity between the terminations and impedance matching approaches used in the differential signal protocols and the test equipment with single ended test equipment can sometimes cause problems. For example, low voltage differential signaling drivers assume a far-end termination of 100 Ohms between the true and complement differential signals with a high impedance to ground potential. When such a driver is connected to typical test equipment with a 50 Ohm single-ended resistance to ground, the driver output signal swing is degraded, the common-mode voltage is reduced, and the signal shape is distorted. Similarly, when a function generator or similar test equipment expecting a 50 Ohm termination to ground drives a low voltage differential signaling receiver having a 100 Ohm termination between its differential inputs, the signals are distorted in shape, level, and swing.